A viscous coupling is a torque transmitting element which functions by means of viscous friction whereby the torque-speed characteristic is dependent upon the rheological properties of the viscous filling medium which is usually a silicone based fluid.
In the past, viscous couplings have usually comprised sets of mutually interleaved parallel annular plates coaxially mounted for rotation within an hermetically sealed enclosure containing a viscous medium in contact with adjacent surfaces of the plates. Alternate plates are connected with respective driving and driven elements such that an angular velocity differential across the coupling causes viscous shearing of the fluid in the annular clearance between adjacent plates to produce the desired speed-torque transmission characteristic. Commonly used silicon based fluids in viscous couplings exhibit a degressive torque response with increasing rate of shear.
It is also known to provide means whereby the plates can be urged into frictional engagement under predetermined operating conditions to increase torque transmission and to provide a series of radially spaced perforations in the plates to permit predetermined variation of the torque transmission characteristics of the coupling to suit particular applications.
However, these viscous couplings of the prior art are relatively slow to transfer torque and do not provide great design flexibility, particularly in applications requiring the transmission of large torques. Furthermore, known viscous couplings are usually highly sensitive to variations in operating conditions such as temperature and fluid viscosity and pressure.